Soil Management and Insect Control

Walter E. Fleming.

Insects are the chief rivals of man for the available food of the world. Intensive agriculture, in which a few kinds of plants may occupy the land in nearly pure stands, provides abundant food and a favorable environment for the insects that attack those crops.

Many insects pass part of their life in the soil. The damage they do to plants by chewing or sucking on the roots and underground parts of the stems may not be noticed until the crop has suffered.

The noxious insects in the soil may be divided into four general groups on the basis of the time they spend in the soil and the kinds and parts of plants they attack.

Examples of these groups are:

1. The white grubs and wireworms that live in the soil for 1 to 3 years and chew the roots and underground stems of a wide variety of plants during the growing seasons.

2. The pale western cutworm and the glassy cutworm, which appear in the spring and attack nearly all plants, except those with hard, woody stems.

3. The root and the seed maggots that appear in the field after the crop is planted but generally are quite restricted in the kinds of plants attacked.

4. The plum curculio, the flea beetles, the sweetclover weevil, and the grasshoppers that spend much of the year below the surface of the soil but do part or all of their damage by feeding aboveground.

The management of soil to control many kinds of insects must be based on a knowledge of their life history and habits. An operation that is effective against one kind of insect might be useless against a closely related kind because of the differences in their cycles of development or behavior.

Tillage, crop rotation, time of planting, fertilization, irrigation, and field sanitation help in our fight against noxious insects. Good husbandry reduces the possibility of infestation by soil-inhabiting insects but does not always control it. A good soil insecticide also may be necessary.

CROP ROTATION often helps reduce damage by insects that attack only a few kinds of plants.

Growing plants of any given kind on the same land year after year produces a condition favorable to the insects that attack that crop.

Chinch bugs are controlled by planting legumes or other immune crops in place of corn, wheat, barley, oats, or rye. A rotation to avoid two successive crops of corn on the same land will do much to prevent injury by the corn root aphid, the seed-corn maggot, the northern corn rootworm, the southern cornstalk borer, and the sugarcane beetle. A 4-year rotation of corn, soybeans, small grain, and clover is recommended in the North Central States to control the European corn borer.

Planting pastureland the first year of cultivation to crops other than corn, rice, or potatoes tends to prevent damage by sod webworms, whitegrubs, wireworms, and cutworms.

A cropping system of wheat following wheat favors an increase in infestation by the hessian fly, the false wire-worm, the winter grain mite, the wheat jointworm, the wheat stem saw-fly, and the wheat strawworm. This cycle of wheat may be broken by planting oats, buckwheat, corn, or sorghum in fields infested by the hessian fly.

Legumes or sorghum are recommended to break the cycle for the false wireworm; corn, sorghum, cotton, or clover for the winter grain mite; rye, barley, oats, or buckwheat for the wheat jointworm; and barley, oats, flax, corn, or mustard for the wheat stem sawfly. Since wheat is apparently the only host of the wheat strawworm, any other crop will break its cycle.

Peanuts, soybeans, velvet beans, and crotalaria provide good food and cover for the adult white-fringed beetle and its grubs. The grasses, including corn and the small grains, are poor foods for the adults. When the succession of crops is a legume followed by oats, corn, and cotton, the population of grubs in the soil does not increase to the extent that much damage is done to susceptible crops.

The native white grubs and those of the Japanese beetle, the European chafer, the Asiatic garden beetle, and the oriental beetle feed on a wide range of garden and field crops and grasses and nursery plants, but they do not thrive in plantings of white-clover, red clover, alsike clover, sweet-clover, alfalfa, soybeans, buckwheat, or orchardgrass.

The proper use of legumes in the rotation cycle in cultivated fields, or in combination with grasses in pastures, reduces damage by white grubs.

The habits of wireworms differ in the various sections of the United States. In the irrigated areas of the Pacific Northwest, the wireworms increase in abundance in cultivated land on which truck crops are grown and in fields of sweetclover, red clover, or small grains. Pasture, if maintained for several years, is detrimental to them. Growing potatoes in short rotation with clover or grain is undesirable.

The best rotation for keeping these wireworms at a low level is 3 or 4 years of alfalfa followed by 1 year of potatoes, and 1 or 2 years of other truck crops, such as sugar beets, corn, beans, or peas. In regions east of the Rocky Mountains, wireworms thrive in land devoted to hay crops or small grains. They are best controlled by rotations that include legumes (alfalfa, sweet-clover, red clover, or soybeans) by not planting susceptible crops two successive years and by clean cultivation.

THE TIME OF PLANTING corn has a great influence on its infestation by some insects. Early-planted corn is damaged less by the corn earworm in the North Central and the Northeastern States, but in the Southern States early corn is more apt to be infested than corn planted later because other attractive plants, such as cotton, are not then available to the moths.

In the North Central and the Northeastern States, corn planted early is usually more severely infested by the European corn borer, but late corn is likely to be damaged by the second brood of the borer. Planting corn in the Southwest as late as is consistent with good agronomic practice is one of the best methods for controlling the southwestern corn borer.

In the Southeast, corn should be planted in April and May, when the southern corn rootworm is the least active. In areas where the southern cornstalk borer is common, planting somewhat later than usual may be a good way to reduce the injury it does.

Early plantings of corn and sugarcane in the Southern States are injured less than late plantings by the sugarcane beetle and by wireworms. In parts of Louisiana where sugarcane is subject to severe damage by wire-worms, planting the cane as early as practical in August gives better stands than plantings made late in September or October.

Beans, corn, peas, and melons may not get past the sprouting stage if the seed-corn maggot is in the fields. Because damage by maggots occurs usually in early spring and in cool, wet soils, it is wise to delay the planting so that the seeds do not sprout until the maggots are no longer active.

A practical and effective way to avoid infestation of wheat by the hessian fly is to delay the seeding in the fall until most of the fall brood of the fly has disappeared. By delaying the sowing, considerable damage by false wireworms, the wireworms, and white grubs also is prevented.

Early-seeded crops make considerable growth before grasshoppers hatch and withstand feeding by grasshoppers better than late-seeded crops. Well-advanced crops are less attractive to the hoppers. Barley, oats, and wheat that have headed can withstand considerable defoliation without serious reduction in the yield of grain.

The early planting of all cotton within a given area and during a short period enables the crop to attain maximum growth and fruit before some insects multiply and spread afield.

PLANTS GROWN IN FERTILE SOIL are usually hardier, healthier, and more resistant to insects than those in poor soil. Barnyard manure, green manure crops, and commercial fertilizers are of value in assisting plants to outgrow damage by insects. The fertilizing of field crops often is recommended in combating infestations of the corn root-aphid, the southern corn rootworm, the hessian fly, the white-fringed beetle, the wheat stem sawfly, the sugarcane beetle, and white grubs.

An important means of controlling the bronze apple tree weevil, the shot-hole borer, the greater shot-hole borer, and the flatheaded apple tree borer is to keep the trees growing vigorously by proper cultivation, fertilizing, spraying, and pruning.

PLOWING UNDER the residues of field crops and other trash is a good practice. Plowing under infested stubble may not kill many European corn borers, but the borers subsequently crawling to the surface cannot find shelter and are killed by exposure to the weather and by natural enemies.

Plowing under remnants of sugarcane, sorghum, and corn in fall or winter in the Southern States will prevent many moths of the sugarcane borer and the southern cornstalk borer from emerging. In southern Arizona, turning the stubble under to a depth of at least 4 inches in the fall and thoroughly topworking the land keep most moths of the southwestern corn borer from emerging.

Fewer than 1 percent of the rice stalk borers live through the winter in stubble that is plowed properly in the fall.

Plowing under wheat stubble as early after harvest as practicable and destroying volunteer growth in the summer and early fall does much to destroy the hessian fly, the wheat jointworm, the wheat strawworm, the wheat stem sawfly, the black grain stem sawfly, and the European wheat stem sawfly.

Plowing under volunteer and second-growth wheat before April and seeding of quick-maturing varieties of spring wheat are recommended in Oregon in years when an outbreak of wheat stem maggot is threatened.

The destruction or killing of cotton plants as early as possible before the first killing frost by mechanical or chemical methods aids in the control of the boll weevil and the pink bollworm. Plowing under the crop residue after the stalks are cut further reduces the numbers of the pink bollworm.

TILLAGE may destroy insects in the soil when they are in a vulnerable position or stage of development.

Plowing fields in which grasshoppers have deposited their eggs may bury the eggs so deeply that the newly hatched hoppers cannot reach the surface. Plowing also may bring the eggs to the surface, where they are killed by exposure to sun and wind. Tillage also loosens the soil so that it is not suitable for grasshoppers to lay their eggs.

The pupae of wireworms, native white grubs, Japanese beetles, white-fringed beetles, the European chafer, and the sugarcane beetle are fragile and easily killed mechanically. Plowing fields that have been in small grains or early truck crops between July 15 and August 15 kills many of the pupating wireworms.